Early lesion detection has been shown to significantly reduce breast cancer mortality rates for women over age 50. Screen-film mammography is widely used for early detection of breast cancer. In screen-film mammography, the breast image is formed by recording, on film, the x-ray intensity distribution exiting a compressed breast after exposure of the compressed breast to a uniform x-ray field. The main purpose of mammography is to detect lesions within the breast.
The breast images, or mammograms, are generally obtained from one of two types of screen-film cassettes. The most common screen-film cassette is made of a single phosphor screen, used as a back screen, in combination with a single emulsion film. High image resolution can be achieved because the number of absorbed x-ray photons decreases as a function of depth into the screen. The second type of cassette uses a double emulsion film sandwiched between two phosphor screens. This type of cassette provides higher x-ray absorption efficiency but has a lower spatial resolution. In both cases, only a single film is used.
Unfortunately, there are a number of limitations associated with the use of screen-film cassettes in mammography which reduce its effectiveness in the detection of breast cancer. Specifically, inadequate exposure latitude of the screen-film cassette is a major problem. The exposure latitude of a screen-film cassette is defined as the exposure interval encompassed within the useful optical density range of the film. Inadequate exposure latitude can adversely affect the lesion contrast on a breast image.
The lesion contrast on the breast image is dependent on the subject contrast and the film contrast, which are independent of each other. Subject contrast is determined by the difference in x-ray attenuation between the lesion and its surrounding area, and is therefore a property of the subject for a given x-ray energy. The film contrast is defined as the slope, or as some function of the slope, of the characteristic curve for a film. The characteristic curve, or the H&D curve, is a plot of the optical film density of the processed film as a function of the logarithm of the exposure which effectuated such density (Corney, G. M., 1979). For the same type of film and processing condition, however, the film contrast depends on the film density. FIG. 1 shows the H&D curves of two hypothetical films. These curves demonstrate the fundamental compromise between the film contrast and exposure latitude. That is, the steeper the curve, the higher the film contrast, but the narrower the exposure latitude.
Due to the low image contrast nature of breast lesions on mammograms, high film contrast is required for accurate diagnosis in mammography. The typical film contrast of mammographic x-ray films (e.g., Kodak Min-R films) is about 3.5 for film densities ranging from about 1.0 to 2.0. The corresponding exposure latitudes of these mammographic screen-film cassettes, however, are only about 40. Unfortunately, the film contrast drops quickly for film densities below 1.0 or above 2.0 (Haus, Arthur G., 1992).
It has been shown (Maidment et al., 1993) that the x-ray exposure level arriving at the screen can vary by a factor of 400 in a single radiography of a breast. In part, this variation can occur because the portion of the compressed breast nearest the chest wall is thicker than the portion of the compressed breast furthest from the chest wall. In addition, there can be large variations in breast composition throughout the compressed breast. Current practice in mammographic screening often uses automatic exposure control (AEC) devices to produce a constant film density (.about.1.5) in the center of the compressed breast. As a result, the film densities around the edge of the compressed breast are much greater than 2.0 due to the excessive x-ray exposure to the screen-film cassette in these areas. Therefore, the film contrast is lower for the edge regions.
A typical radiographic phosphor screen is made of a layer of phosphor coated on a support layer. The structures of conventional radiographic phosphor screens are configured so that light emitted from the phosphor side is optimized for intensity and/or image resolution. For example, typical phosphor screens used in chest radiography have a layer of reflection material between the phosphor layer and support layer to reflect light emitted toward the support side back to the phosphor side. This is to increase the light intensity output from the screen. In mammography, however, conventional phosphor screens are made of a phosphor layer coated on a partially absorbing support layer to absorb the light emitted toward the support side. This is to improve spatial resolution by reducing the light that could be reflected back to the phosphor side. In both cases only light emitted from the phosphor side of the screen is used to expose a film.
The use of more than one film in a single x-ray cassette has been explored for use in mammography (Greshon-Cohen, 1960; C. C. Wyatt, 1980) and other areas of radiography (Sanada et al., 1991; McLean, 1996; Trauernicht, 1997). The critical problem in these systems is that x-ray exposure available for one film is significantly less than that for the other film(s). This difference in x-ray exposure level leads to a difference in x-ray quantum noise levels so that the imaging performance of the film which uses the lowest x-ray exposure may be limited by noise. The use of light emerging from both sides of a self-supporting phosphor layer has been disclosed for use in general radiography (Komaki et al., 1983), where each film is exposed by light emerging from at least two phosphor screens located on either side of the film. These screen-film combinations, used in general radiography, require on the order of 60-100 kV, which requires thick phosphor screens to adequately interact with a sufficient fraction of the x-rays. In addition, the quality of the resulting film images can suffer from image noise due to fluctuation in the number of x-rays absorbed in each of the multiple screens, as well as high levels of Swank noise [10] due to significantly different amounts of light incident on the film for each of the multiple screens. Furthermore, in order to have self-supporting phosphor sheets, Komaki et al. disclose the use of phosphor sheets having thicknesses in the range of 70-300 .mu.m and preferably 100-150 .mu.m, which is also consistent with the requirements imposed by the high-energy, 60-100 kV, x-rays as discussed above.
In mammography, mis-diagnosis frequently occurs when a breast lesion is surrounded by dense fibroglandular tissue (Skubic and Fatouros, 1989; Ma et al., 1992). In this case, the subject contrast is very small due to the subtle difference in x-ray attenuation between the lesion and fibroglandular tissue (Johns and Yaffe, 1987). Additionally, the lower x-ray penetration in these regions can cause low film densities and therefore low film contrast. The low subject contrast combined with the low film contrast makes proper diagnosis more difficult. Consequently, some lesions can go undetected, or in the case of an ambiguous diagnosis the patient is often required to undergo reexamination. This results in additional exposure to ionizing radiation as well as additional psychological stress.
The detection of breast cancer is particularly problematic for young women whose breasts typically have high fibroglandular content. Therefore, in the detection of lesions, higher than normal film contrast is particularly desirable. Unfortunately, with the existing state of the art cassettes, this would correspond to an unacceptably low exposure latitude. There is therefore a need for a cassette which can provide higher than normal film contrast with no corresponding unacceptable drop in exposure latitude. Such a cassette would be particularly advantageous in the routine mammographic scanning of women under the age of 50.
It is therefore desirable to develop new methods and screen-film cassettes with wide exposure latitude and/or high film contrast in order to improve the diagnostic accuracy in early breast cancer detection. Such a development could further reduce breast cancer mortality for all women as well as reduce the need for reexamination of the patient and thus reduce patient exposure to ionizing radiation.